Aqueous environments, with the exception of pure water, are electrically conducting due to the presence of solvated ions therein. For example, the solid sodium chloride (NaCl) readily dissolves in water to produce positively-charged solvated sodium ions (Na+) and negatively-charged solvated chloride ions (Cl−). When an electric field is present within the aqueous solution, those ions migrate, producing electrical conduction.
A need to provide electrical energy, power, or electrical signals through electrical conductors that are in contact with, themselves pass through, or are generally proximal to aqueous environments arises in many applications. For example, it may be necessary in some applications to supply electrical energy or power to, or communicate with, a sensor, actuator, or other electrically-powered or electrically-responsive device (which hereinafter may simply be referred to as a ‘device’) on the other side of or in a wet or damp environment. Examples of such applications include powering underwater lights and providing power through underwater cables and electrical connectors that operate in liquid-contacted or submerged environments. Further, operation of certain devices, e.g., an immersion water heater, or certain types of ink jet printheads used with aqueous ink, requires the combination of water and the supply of electrical energy, power, or signals.
In some further applications, there is also an occasional need to provide electrical energy, power, or signals through electrical conductors that are adjacent to liquid environments comprising non-aqueous liquids, in which one or more salts have been dissolved. Examples of such applications again include supply of electrical energy to many forms of ink jet printheads, such as piezoelectric, thermal, and electrostatic printheads.
The present disclosure generally refers to both aqueous and non-aqueous liquids in which one or more salts have been dissolved as electrolyte solutions. A conductor, as referred hereinafter, generally encompasses any material through which electrical energy, power, or signals can be passed. For example, conductors and conducting electrodes, may be made from copper, aluminium, silver, gold, lead, nickel, silicon, carbon, titanium, platinum, mercury or organic materials such as dilithium benzenediacrylate. It should be noted that although silicon is normally classified as a semiconductor, undoped silicon has a typical resistivity of 10 Ωm at room temperature and so behaves as a conductor in this context.
Hereinafter, the present disclosure refers to components, devices, systems, and assemblies to which a power source system supplies electrical energy, power, or signals as presenting an electrical load (or, simply, ‘load’) to the power source system. The nature of the load depends upon the nature of component, device, or system. For example, devices commonly presenting a primarily capacitive load include piezoelectric actuators such as those used in piezoelectric ink jet printheads.
Electrical energy, power, or signals can be supplied to devices using, for example, electrical conductors (e.g., electrodes) by the application of an electrical potential difference (voltage difference) between two or more of such conductors. The value of the required potential difference depends on the nature of the device to which the electrical energy, power, or signals are supplied. If those conductors are in contact with an electrolyte solution (for example, if they are partially or fully immersed in the electrolyte solution) and the potential difference required for the satisfactory supply of electrical energy, power, or signals to the device exceeds the potential difference at which electrochemical reactions can occur in the electrolyte solution, known as the overpotential, an electrical current flows in the electrolyte solution between the conductors.
Different electrochemical reactions may occur in a given set of conductors and electrolyte solution. Each such electrochemical reaction may be associated with a given set of conductors and electrolyte solution and has its own corresponding overpotential. This disclosure refers to the smallest overpotential between two conductors taken from a given set of conductors and electrolyte solution as the threshold overpotential or Vop.
For example, the overpotential to create hydrogen and oxygen on platinum electrodes is 1.11V and the overpotential to create hydrogen and chlorine on platinum electrodes is 0.26V. Therefore, for an aqueous electrolyte containing chloride ions, the threshold overpotential with platinum electrodes is 0.26V. In other words, the value of the threshold overpotential depends on the specific composition of the conductors and the electrolyte solution. Typically however the threshold overpotential for a given set of conductors and electrolyte solution is in the range 0.2V-2V.
Currents flowing from one conductor to another through the electrolyte solution as a result of an applied potential difference exceeding the overpotential are parasitic in the sense that they consume at least some of the electrical energy or power otherwise delivered to the device (so wasting energy). They are also indicative of a potentially undesirable series of electrochemical reduction and oxidation reactions being present.
Evolution of gaseous hydrogen and oxygen, and more generally, the oxidation of the material at the anode and the reduction of the material at the cathode, are examples of the electrochemical reactions that may occur in aqueous electrolyte solutions. Such electrochemical reactions are often undesirable in systems designed to provide electrical energy, power, or signals to devices in contact with or partially or fully immersed in the electrolyte solutions over long periods of time. For example, evolution of gases, such as hydrogen, adjacent to metals and piezoelectric materials can cause degradation of desirable material properties of such materials. Similarly many otherwise-desirable conductor materials, such as copper, become oxidised at the anode and ultimately fail, provided a supply of ions giving rise to conduction in the electrolyte solutions remains.
Conventionally, to prevent both waste of electrical energy and undesirable electrochemical reactions, the conductors are shielded from the electrolyte solution with a waterproof and insulating layer, such as a rubber or elastomer coating. This approach typically, though not always, works well for a large static electrical cable. However, such an approach is often impractical when cable flexibility is required, or in applications in which an electrical connection needs to be made and unmade within the electrolyte solution environment. The coating/shielding approach is also often impractical when the electrical conductor is placed adjacent to the electrolyte solution itself, as is the case, for example, with some designs of ink jet printheads. In applications requiring the electrical conductor to be placed adjacent to the electrolyte solution, the protection is often provided by applying a thin insulating coating to the electrical conductors, for example, a coating formed of soft polymeric materials, such as Parylene, or hard materials, such as silicon nitride (Si3N4). Unfortunately, the above described coating and shielding approaches fail in certain circumstances, thus rendering them unsatisfactory for at least some applications.
For example, mechanical damage to the conductor or its coating layer results in the removal of the coating layer from the conductor, and/or pinholes in the coating layer, and/or regions of the conductor that are not coated by the insulating layer. This in turn, will expose the conductor to the electrolyte solution, thereby allowing parasitic currents and undesirable electrochemical reactions to occur.
In addition, if the coating layer is thin and subject to extreme electric fields, such as of the order 107 V/m, local imperfections within the coating layer can allow the dielectric strength of the insulator (coating layer) to be exceeded in the region of the imperfections. Consequently, pinhole-like conducting paths between the electrolyte solution and electrical conductor will open up, thereby allowing parasitic currents and undesirable electrochemical reactions to occur.
Further, while an electrical conductor is being coated with an insulating layer, a small number of water molecules and solvated ions can become trapped under the coating. This would create a thin film of electrolyte solution in-between the conductor and the insulating layer. When a potential (voltage) difference is created across the thickness of the thin film of the electrolyte solution, the water molecules and ions in the film are induced to undergo electrochemical reactions, thereby causing hydrogen and oxygen bubbles to form. These bubble formations can be problematic for thin coatings.
If the coating is thick however, repeated flexure of the conductor/coating assembly can induce cracks to be formed within the coating, exposing the conductor to the electrolyte solution and/or opening pinhole-like conducting paths between the electrolyte solution and electrical conductor. This in turn, allows parasitic currents and undesirable electrochemical reactions to occur.
In many devices, the electrical conductors are supported by a metallic shell that could for example take a form of an armoured sheath, a metal container, a chassis, or, in a case of an ink jet print head, ink supply pipes. These components may be connected to ground potential or allowed electrically to float. They can also form a part of the electrical circuit. For example, the nozzle plate of an ink jet print head, which can be made of stainless steel, electroplated nickel, silicon, or metallised polyimide, or the substrate can act as a part of the electrical circuit. Leaving the potential of such components uncontrolled can lead to destructive electrolytic currents at the conductor-electrolyte solution interface.
When a potential difference greater than the threshold overpotential is applied between two conductors adjacent to a common electrolyte solution, the following sequence of processes (stages) occurs:    1. A layer of electrical charge forms on the surface of each conductor, according to the applied potential difference.    2. Ions in the electrolyte solution are attracted towards and closely approach the surface of each conductor to form a layer of counter charge, thereby creating a charged double layer capacitance adjacent to the surface of each conductor. This disclosure refers to the characteristic time for formation of each charged double layer as its ‘double layer charging time.’    3. A set of reversible reactions takes place on the surface of one or both conductors, for example, the reversible formation of metal hydrides at the cathode.    4. A set of irreversible reactions associated with the net electrical charge flow into the electrolyte solution from the conductor(s) occurs on the surface of one or both conductors, such as the creation of hydrogen bubbles at the cathode and/or the irreversible oxidation of the metal electrode at the anode. At this stage, the irreversible degradation of the conductors may be caused by the ionisation and dissolution of the metal electrode into the electrolyte solution, or the break-up of the metal surface as the oxide layer grows deeper into the anode structure. These redox reactions are often also associated with electrolysis of the electrolyte which can cause other forms of device failure.
Following the establishment of charge on the surface of each conductor in stage (1), the time taken for stages (2)-(4) varies with and depends on the conductor material, the conductor geometry, and the ionic content and pH value of the electrolyte solution. The double layer formed adjacent to the surface of each conductor can sustain only a certain maximum potential difference (at which the double layer may be considered fully charged) whose value is determined by the nature of the electrical conductor materials and of the electrolyte solution. The threshold overpotential, above which the irreversible reactions of stage (4) occur, can therefore be understood as the sum of the absolute values of the maximum potential difference that can be sustained by the double layer formed on the first conductor and the maximum potential difference that can be sustained by the double layer formed on the second conductor.
As mentioned, the supply of electrical energy to many forms of ink jet printheads, including certain types of piezoelectric, thermal, and electrostatic printheads, requires the electrodes supplying electrical energy to actuation components to be adjacent to the electrolyte solution. A ‘shared wall’ piezoelectric ink-jet printhead for example has an array of actuator walls separating chambers that in use are either (i) all filled with liquid to be ejected or (ii) are alternately filled with liquid and unfilled. Unfilled chambers may be referred to as ‘dummy’ chambers. In a ‘shared wall’ piezoelectric ink-jet printhead, electrical conductors in the form of electrodes (i.e., actuator electrodes) are arranged on one or more of the piezoelectric actuator components, and have a common fluid pathway and a support structure. The support structure typically contains other metallic, and thus conductive, components such as ink feed tubes which are also in close contact with the ink.
U.S. Pat. Nos. 6,106,092A, 6,193,343B1, and US2009/0073207A1 describe specific schemes for driving a shared wall piezoelectric printhead having multiple piezoelectric actuator components and a common liquid supply. U.S. Pat. No. 6,106,092A describes the ejection of droplets effected by applying positive and negative potentials of short duration to first liquid-wetted actuator electrodes whilst holding neighbouring liquid-wetted electrodes at ground potential. In this way, drive pulses having sharp edges and comprising both positive and negative potential differences between liquid-wetted electrodes are created. U.S. Pat. No. 6,193,343B1 describes with reference to FIG. 13 how such drive pulses, comprising both positive and negative potential differences, can be created using positive-only potentials referenced to ground potential. Namely, first, positive drive pulses are applied to first actuator electrodes whilst their neighbouring electrodes are held at ground potential. Then, positive drive pulses are applied to the neighbouring actuator electrodes whilst the first actuator electrodes are held at ground potential. US2009/0073207A1 discloses using such positive-only drive pulses with ink jet printhead in which all chambers are ink-wetted.
The use of positive-only (or negative-only) drive pulses is particularly likely to lead to a potential difference between the average potentials of the array of liquid-wetted electrodes on a piezoelectric ink jet printhead and liquid-wetted metallic support components greater than the threshold overpotential. Further, according to U.S. Pat. Nos. 6,106,092A, 6,193,343B1, and US2009/0073207A1 the duration of the negative potential differences between adjacent liquid-wetted electrodes is approximately twice as long as the duration of the positive potential differences whilst their magnitude is the same. The magnitude of the average potential difference between adjacent liquid-wetted electrodes during trains of pulses (such as those shown in FIG. 5 of U.S. Pat. No. 6,106,092A) is therefore non-zero. In either case, electrochemical corrosion therefore results if any dielectric overcoat layers on the electrodes have pinholes or other imperfections. To avoid such corrosion, only substantially non-conducting liquids can be used.
Another example of printheads is inkjet heads in the form of Micro-Electro-Mechanical Systems (MEMS) such as those described in EP0511376B1. In these inkjet printheads, the material adjacent to the ink solution is silicon, and its electrical conductivity is such as to enable passing of electrical currents from the electrodes to the ink. Thus, the silicon material acts as a conductor in this context. The enabled electrical currents can however cause undesirable electrochemical redox reactions.
Therefore, there is currently a need for an improved method and apparatus for supplying electrical energy, power, or signals to devices whose operation requires transmission of electrical energy, power, or signals through electrical conductors that are proximal to, in contact with, or pass through an electrolyte solution environment. In particular, it is desirable to suppress or fully avoid irreversible reactions that may occur on the surface of one or more conductors as a result of an electrical charge flow from the conductor(s) into the electrolyte solution. More specifically, it is desirable to prevent or reduce the irreversible degradation of the conductors that may be caused by the ionisation and dissolution of the conductor (such as a metal electrode) into the electrolyte solution, or the break-up of the metal surface as the oxide layer grows deeper into the anode structure. For example, it is desirable to prevent the creation of hydrogen bubbles at the cathode and the irreversible oxidation of the metal electrode at the anode.